I think there is some science, and a fair bit of art in this!
Clem has given the method which I use (and you will find this in Camerons queries), this to me does take some account of the roach. Glad to see you are still using what I suggested to you all those years back Clem!
The CLR ( centre of lateral resistance) has fascinated me for years. I too had assumed it to be just a bit forward of the rear axle. That is what the very old "Landsailing RC Models to the Big ones" book reckoned. When Bill Finch came over in 82 he persuaded me to check this out by dragging yachts sideways (also mentioned in the Cameron queries) on pea gravel or wet grass. Let's just say there is a lot to be learned here. Weight distribution, tyre pressures, wheel sizes all affect this. Sure it's only what you get at "no speed" but it's possibly better than an assumption that it will be 2" forward of the axle. Also explains a bit about where you put your weight while sailing upwind/downwind to enhance performance?

Fibreglass over timber is how "Wildfire" seat was made.
I held the plywood together with plastic cable ties through small holes in the edges, and once the inside was glassed I cut away the excess plastic ties and did the outside. simple project

This method was a radical change in boat building, it is called "stich and glue" and many small sail boats and canoes are designed and built this way, very efficent and doesn't require an internal frame.

I have just had a look at the D4. It seems like a good project. It will teach you a lot about stitch and glue, and a little about F'glassing. It's a good looking little boat, but not spectacularly historic looking enough IMHO to need bronze fittings.
If I built one it would certainly use the more available and cheaper ss and ali fittings.
FWIW I earned my living for a number of years in the glass industry, it is not hard to do, just gets a bit tedious at times. Trying to use the available products more wisely is my fascination with the stuff.

Sheathing-personally I am not a fan of chopped strand for this, partially because of the difficulty / tedium of fairing it later which is a little easier with a fabric type.
With stitch and tape I now cut my own tapes with the fibres at 45 degrees so that you have multiple crosses over the join rather than very few in the direction along the joint and lots at 90.
Cable ties are very easy, I don't think the size of the holes matters. Do one side (the "non-nut" side) let it harden then cut the nuts off and glass the other. I have used mono fishing line, but you have to learn to tie fishing knots. Copper allows you to put them in, twist, and then bend again to tighten even more. The last canoe I did is 12 feet long, preglassing weighs 4.1 kg. Its just joined with Gorilla glue and dry wall screws, glassed on the inside, then the screws all removed, the outside hit with the belt sander and glassed with a 4oz boat cloth. Now it weighs just over 8kg! I do not believe any mechanical fastenings are needed to join panels. The last water yacht now 15 years old has done 2 worlds champs, and 100s of hours sailing and has not one screw or nail in the structure except to hold fittings on. No failures at all, so it must be too strong! (Needed more lead to bring to min weight than technically allowed too)
You pays your money.....

I intend using stitch and glue for my mini seat as I am familiar with the process
having built three boats that way
I have always used copper wire in the past cutting the cloth on the bias for the taping as it is double the strength that way but double bias tape can be bought these days One trick I learnt was radiusing off the corners of the panels before wiring them up it makes fairing up the panels easier as the panels are not trying to fit corner to corner and they can be moved into position better

landyacht said...
[br the mini explorer seat (all glass ) out of the mold is 4kg.

That with moulded comfort sounds good to me.

Fear not landyacht I cant imagine anything ridiculously sexy coming out of my shed
It will be grass roots basic
cant speak for the others though